This invention relates generally to sprue bushings for injection molding machines and, more particularly, to internally-heated sprue bushings in which the sprue bushing inner core has an outer coating of highly thermally conductive material and to the method of making such sprue bushings. In this regard, one important aspect of the present invention is directed to an internally heated bushing assembly which is especially suited to handle pressurized melt, which assembly includes a unitary bushing casing-core element having an integral outer sidewall, endwall and elongated copper-clad inner core having a melt running passage therethrough.
Sprue bushings are commonly used in injection molding to provide a heated flowpath for conveying molten plastic or melt from an injection molding machine manifold or nozzle to an injection mold cavity opening. Typically, these sprue bushings may include a separate bushing casing portion housing a heating element and a separate, inner bushing core portion through which the melt flows. The bushing core is usually disposed in the center of the bushing casing and extends axially through the casing between the bushing inlet and an outlet end thereof with one end of the core projecting through a hole in the casing. Other sprue bushings such as that shown in U.S. Pat. No. 4,882,469 have an integral casing-core construction wherein the casing and core are formed from one piece with an annular cavity formed therebetween which accommodate a preformed tubular heater element. In either of such sprue bushing constructions, the inner core is surrounded by a tubular or spiral heating element set in a particulate, compacted refractory material. The refractory material insulates the heating elements and conveys or transmits heat from the heating element(s) to the center core portion of the bushing.
Although the refractory material transmits heat adequately to the bushing center core, it is desirable to increase the effectiveness of heat transfer from the heating element to the inner core. This is accomplished in the present invention by providing the inner core with an outer layer of a highly thermally conductive material, such as copper or an alloy thereof. The outer layer of the highly thermally conductive material provides for more efficient heating of the inner core melt runner passage for substantially the entire length thereof. Significantly, the outer thermal layer, or cladding, of copper may extend between the point where the bushing inner core meets the head member at the melt inlet of the sprue bushing and the rear inner wall of the bushing casing near the bushing outlet. The transfer of heat through the outer thermal layer or cladding of copper or other highly thermally conductive material is effectively accomplished along substantially the entire length of the center core, thereby effectively improving the heat distribution along the melt runner passage.
The present invention provides the above-mentioned benefits and is directed to a novel sprue bushing assembly in which the bushing inner core or central melt runner passage has an outer layer of a highly thermally conductive material applied thereto, such as copper. As such, some of these sprue bushing assemblies are characterized by a unique bushing casing-core element which includes an integral outer sidewall, endwall and elongated inner core having a melt runner passage extending therethrough which inner core has added thereto an outer layer, or sleeve, of high thermally conductive material. Other of these sprue bushings are characterized by a unique bushing casing-core element which includes an integral outer sidewall and endwall, and a separate elongated inner core joined to the bushing endwall, having a melt runner passage extending therethrough in which the inner core has an inner melt passage portion and an outer thermally conductive portion.
In a sprue bushing assembly incorporating the principles of the present invention, a cylindrical metal blank is provided with a cavity axially extending within the blank. An elongated inner core member, which can either be integrally formed with the bushing casing endwall or separately joined thereto, is spaced apart from the outer wall of the bushing casing. The cavity contains a sleeve or annular space filled with a high thermally conductive material which closely abuts the outer surface of the inner core member and also contains at least one heating element in the form of a hollow preformed electrically non-conductive sleeve which encircles the inner core thermally conductive sleeve. Open spaces which occur in the casing cavity between the heating sleeve and bushing casing are filled with a powdered or particulate refractory material to form a heater-bushing blank assembly. When filled, the heater-bushing blank assembly is then swaged and reduced down to a preselected uniform diameter, thereby forming all of the components which fill the casing cavity, i.e., the thermally conductive sleeve, the refractory material and preformed heating element, into a unified mass which is substantially free of voids. A head member containing electrical conduction and temperature sensing wires to the heating element is then added to the top portion of the bushing-heater assembly and the cap then firmly welded to the bushing assembly. An opening is drilled through the bushing core to provide a melt flowpath. The outlet end of the bushing is then finished to form a final bushing outlet tip.
Accordingly, it is a general object of the present invention to provide a method of manufacturing an improved sprue bushing from a single metal blank having improved heat transfer means operatively associated with the inner core and wherein the inner core is integral with the bushing casing which eliminates melt leakage and heater element burnout.
Another object of the present invention is to provide an injection molding sprue bushing in which the bushing core has a copper outer surface in contact with a multi-stage heating element for supplying heat to substantially the entire length of the bushing core.
A further object of the present invention is to provide a unitary sprue bushing wherein the bushing includes an outer annular casing, an inner core axially extending through the bushing casing, a copper sleeve formed on the bushing inner core, and an annular cavity encircling the inner core and separating the bushing casing from the inner core which cavity contains at least one preformed non-electrically conductive sleeve.
A yet further object of the present invention is to provide an internally heated sprue bushing having an inner core with an outer copper sleeve which is integral with the bushing casing and in which a thermowell is received in contact with the inner core and copper sleeve.
Still another object of the present invention is to provide a sprue bushing for injection molding having an outer bushing casing, an inner core axially extending through the bushing casing, the inner core having an outer coating of highly heat conductive material and an inner melt runner passage axially extending through the bushing inner core, the bushing further including an annular cavity disposed between the inner core and bushing casing, the cavity having two heating element means in the form of preformed sleeves axially aligned therein, each heating element means providing heat to a different portion of the inner core and its outer coating of the sprue bushing.
These and other objects, features and advantages of the present invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings wherein like reference numerals refer to like parts.